Melting in
the shallow mantle is well documented. It is caused by the adiabatic ascent of
a material or by the addition of “flux” such as water and/or carbon dioxide
that reduces the solidus. Melt density and melt morphology (i.e., the dihedral
angle) are well known. Consequently, it is possible to interpret some
geophysical observations in terms of the presence of melt: in most cases,
geophysical anomalies are difficult to attribute to the presence of melt unless
the melt geometry is unusual (e.g., zero dihedral angle).
Melting can
also occur in the deep mantle particularly across the mantle transition zone
and in the D” layer. Recent experimental studies show that melting is
ubiquitous in the deep mantle (deep upper mantle and the lower mantle), but the
geochemical and geophysical consequence of melting in the deep mantle is
largely unknown. In most cases, melting in the deep mantle is “flux melting”
assisted by the volatiles. I will summarize the current status of studies on
melting in the deep mantle with the focus on the conditions for melting,
chemical composition of the melt and the melt density with the focus on the
role of water. Water-induced melting in the lower mantle is extensive and in
almost all areas in the lower mantle melting is difficult to avoid unless other
materials that dissolve volatiles exists (e.g., metallic Fe). The composition
of the melt produced in the lower mantle is (Mg,Fe)O-rich as opposed to the
melt produced in the shallow mantle (SiO2-rich). Consequently, the
deep mantle melting will affect the chemical evolution of Earth quite
differently than the shallow mantle melting. However, two key parameters,
namely the density and the dihedral angle, are poorly constrained. A review of
current status and a discussion on the future directions will be provided.

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